Field of the Invention
The present invention relates to an optical waveguide sheet, an edge-lit backlight unit and a laptop computer.
Discussion of the Background
Liquid crystal display devices in widespread use have been in a backlight system where light emission is executed by irradiating onto a liquid crystal layer from the rear face. In this system, a backlight unit such as an edge-lit backlight unit and a direct-lit backlight unit is mounted on the underside of the liquid crystal layer. As shown in FIG. 4, such an edge-lit backlight unit 110 generally includes a top plate 116 disposed on the backmost face of a liquid crystal display unit, a reflection sheet 115 disposed on the front face of the top plate 116, an optical waveguide sheet 111 disposed on the front face of the reflection sheet 115, and a light source 117 that emits rays of light toward the end face of the optical waveguide sheet 111 (see Japanese Unexamined Patent Application, Publication No. 2010-177130). In the edge-lit backlight unit 110 shown in FIG. 4, rays of light that are emitted by the light source 117 and enter the optical waveguide sheet 111 propagate in the optical waveguide sheet 111. A part of the propagating rays of light exit from the back face of the optical waveguide sheet 111, are reflected on the reflection sheet 115 and enter again into the optical waveguide sheet 111.
In laptop computer having such a liquid crystal display unit, in order to enhance its portability and user-friendliness, a reduction in thickness and weight is required, leading to a requirement also for a reduction in thickness of the liquid crystal display unit. In particular, in a thinner type laptop computer referred to as Ultrabook (registered trademark) in which the thickness of the thickest part of its housing is no greater than 21 mm, it is desired that the thickness of the liquid crystal display unit is about 4 mm to 5 mm, and thus, further a reduction in thickness of the edge-lit backlight unit incorporated into the liquid crystal display unit has been desired.
In regard to an edge-lit backlight unit 210 of such Ultrabook, as shown in FIG. 5, an edge-lit backlight unit is also proposed in which a reduction in thickness is attempted by dispensing with the reflection sheet 115 as shown in FIG. 4. The edge-lit backlight unit 210 shown in FIG. 5 includes a metal top plate 216, an optical waveguide sheet 211 overlaid on the front face of the top plate 216, and a light source 217 that emits rays of light toward the end face of the optical waveguide sheet 211, in which the front face of the top plate 216 is finished by polishing and functions as a reflection surface 216a. In this example, the rays of light that are emitted by the light source 217 and enter the optical waveguide sheet 211 propagate in the optical waveguide sheet 211, and a part of the propagating rays of light exit from the back face of the optical waveguide sheet 211, are reflected on the reflection surface 216a disposed on the front face of the top plate 216, and enter again the optical waveguide sheet 211. Thus, in the edge-lit backlight unit 210 shown in FIG. 5, the front face of the top plate 216 corresponds to the reflection surface 216a, and the reflection surface 216a can serve as the reflection sheet 115 shown in FIG. 4. Therefore, the edge-lit backlight unit 210 dispenses with the reflection sheet 115, leading to achievement of a reduction in thickness of the liquid crystal display unit.